1. Field of the Invention
The present invention relates to protection circuits, devices that incorporate protection circuits, and methods of protecting circuits and devices from damage due to faults or low impedance during power up.
2. Background of the Related Art
Many printed circuit cards use an electronic circuit breaker (ECB) protection circuit that protects against faults or low impedances that may occur while the printed circuit card is energized. These protection circuits usually have at least one threshold current limit, above which, the protection circuit removes energy from the intended device load and turns off.
FIG. 1 is a circuit diagram of a prior art electronic circuit breaker 10 for controlling the amount of electrical current provided to a device load 20. The power supply (not shown) provides an input voltage (Vin) 11, which is assumed to be a positive voltage (possibly 12V) that provides power to the device load 20 through the ECB 10. The ECB 10 consists of an ECB Control Circuit 12 illustrates as an integrated circuit (IC1), a current sense resistor (Rsense) 13, a transistor pass element (Q1) 14 such as an n-channel MOSFET, and a soft start timing capacitor (C1) 15 coupled to ground.
When an Enable signal is asserted to the Enable pin (EN) 16 of the ECB control circuit 12, the ECB control circuit 12 causes a gate drive output 17 to generate a charging current to the soft start timing capacitor 15, causing the voltage on the soft start timing capacitor 15 to rise and the transistor pass element 214 to begin to turn on. As the transistor pass element (Q1) 14 turns on, the output voltage (Vout) 26 on the device load 20 begins to rise. Assuming that the device load 20 has no detectable fault, the transistor pass element 14 continues to turn on until the output voltage (Vout) 26 equals input voltage (Vin) 11 from the power supply minus the voltage drop across the current sense resistor (Rsense) 13 and the transistor pass element (Q1) 14.
The ECB control circuit 12 monitors the amount of electrical current supplied to the device load 20 using current sense inputs 18 that measure the voltage drop across the current sense resistor (Rsense) 13. If the load current exceeds a prescribed current threshold indicating that a fault exists in the device load, then the ECB control circuit 12 will discharge the voltage across the soft start timing capacitor (C1) 15, turning off the transistor pass element (Q1) 14 and removing power from the device load.